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ICHIGAN

2-1 MINING AND MILLING

Uranium-bearing ore contains both U238 and U235 in the pro of about 140 to 1, together with their respective decay pr Uranium-235 and its daughters are present in such small a that they do not constitute a health hazard. Radon gas, an emitting daughter in the U238 decay series, and its daught the only significant radioactive health hazard encountered in 1 Radon emanates from exposed mineral surfaces in a mine and into the mine air; studies 2 in mines have shown emanation rat range from 9 X 109 to 2 X 10" atoms of radon per minute per 1 ft of mine volume, or from 5 X 10-7 to 2 X 10-5 curies/m 1000 cu ft. This radon, together with its daughters, constit radiation hazard to miners breathing the air.

In poorly ventilated mines the concentration of these day in the mine air frequently rises to highly significant levels method for controlling these concentrations consists essenti adequate ventilations to continually and thoroughly ventil areas of a mine with an ample amount of fresh air. The s of this radiation hazard to miners has not always been appre In recent years, however, appropriate standards for maximu missible concentrations have been developed and adopted by tory agencies. Mining companies, assisted by these agencies improved ventilation practices and thereby have generally b this hazard under control.

Domestic uranium-ore mills produce 2 in the aggregate up to lb uranium oxide, U3O8, per day containing about 12.2 curies At secular equilibrium each daughter also contributes 12.2 The disposal of these wastes, whether contained in ventilati water, or waste rock, requires controls.

3

The process for extracting uranium from the ore generally in crushing, grinding, leaching, and separation of the leaching from the barren tailings. The mills are classified as acid or line depending on whether sulfuric acid or sodium carbona sodium bicarbonate are used in leaching. In the acid-leach p the leaching liquor is separated from the tailings by counter decantation (CCD), and the dissolved uranium is recovered an centrated from the leach solution by ion-exchange resins or o liquid extractants. The separation of solids from the leachin tion must be complete in each process except in one adaptation ion-exchange recovery method, the resin-in-pulp method, in whi ion-exchange resin is dispersed in a desanded but unclarified le solution still containing from 4 to 15% fine solids. Acid-leac therefore are classified as acid leach-ion exchange (IX), acid resin in pulp (RIP) or acid leach-solvent extraction (SX). Ur

is removed from the resin or solvent by stripping with an aqueous solution and is then precipitated with alkaline reagents. In the alkaline-leach process, the leach liquor is clarified by filtration and is precipitated with caustic soda.

The barren tailings that are separated from the leaching solution comprise the insoluble portion of the ore. These tailings contain only a small percentage of the original uranium content of the ore, but they contain virtually all of the radioactive decay products of uranium that were present in the ore (except the gas radon). About 90 lb of tailings result from each 100 lb of ore. They are disposed of by impounding them at suitable sites on the mill property. The tailings are conveyed from the mill to the impounding area, or tailings pond, where they settle out and are retained; the clarified water is collected and recycled. The clarified water contains waste chemicals and a small soluble fraction of some of the radionuclides, notably Ra226, Th23o, and minor amounts of uranium. A relatively small fraction of this recycling solution is regularly discarded to compensate for the amount of fresh water necessary to the mill operation.

230

As uranium ore is processed, many of the daughter elements disappear because of their short half-lives; some long-lived nuclides, however, survive and constitute a potential radioactive pollutant. The most important of these is radium. The average grade of ore contains as much as 0.7 mg of radium per ton. Analyses of samples of clarified-tailings pond water taken from various uranium mills from 1957 to 1959, summarized in Table 2.1, show that the uranium concentration in two of six effluents exceeded the others by factors of 10 or more. The permissible radium specification for water was exceeded in all cases, the Th230 concentration was probably too high in two of five cases, and the Th234 specification was not exceeded at all. Dissolved-radium levels in the streams where tailings have been discharged directly have been reported.4-6 Levels encountered in various Colorado streams in 1950 and 1955 are given in Table 2.2, and those observed in the Animas River in 1958 and 1959 are given in Table 2.3. Calculations using the data in Table 2.3 show that the average daily discharge of suspended solids by the uranium mill in question contained 171μc of gross-alpha activity, 160 lb of uranium, and an average of about 0.30 mg with a maximum of about 0.80 mg of radium. The significance of these values depends upon the dilution volume available in the Animas River and upon the retention and subsequent release of this material from the suspended solids to the water environment. Plant changes in ore processing made between the sample collections of 1958 and those of 1959 were quite effective in reducing radium and gross-radioactivity levels in the stream. The

§Alpha activity above 50 percent dissolved; beta activity above 12 percent dissolved.
Suspended solids very high (3150 and 4400 mg/liter); most of gross alpha and beta activity in suspended

form.

uranium-ore processing mills have continued to introduce substantial improvements in processing and in controlling effluent releases.

Very little thorium has been mined in the United States; thus disposal of these wastes has not been a problem. Wherever thorium is mined, however, its more energetic decay products and their resultant lower biological tolerances might make the problem of radioactive-waste disposal from thorium-ore processing more serious than that of uranium.

Airborne dusts originate in various steps in uranium-milling operations. Dust containing all constituents of the ore is produced in large quantities in the sampling, grinding, and crushing areas and requires ventilation control. The primary health hazard is silica dust. In the chemical-processing sections of the mills, highly toxic compounds, such

as hydrogen sulfide, arsine, or acid gases as well as uranium dust, ma be generated under some circumstances.

All aspects of the radiation hazards in uranium-milling operations as they may affect employees or the general public, are subject to th regulations of the Atomic Energy Commission, Code of Federo Regulations, Title 10, Parts 20 and 40. All other health and safet hazards of milling, including the problems of nonradioactive pollutio of natural wastes and of air, are subject to the pertinent regulation of state and other public health agencies.

As shown in Table 2.4, the 26 uranium-ore processing plants are lo cated within an eight-state area." These plants have capacities rang ing from 50 to 3,500 tons of ore per day and a total daily mill fee capacity of about 21,000 tons.

In summary, radon gas, together with its radioactive daughters is the most hazardous radioactive contaminant in the mines. Ore milling operations confine most of the radioactive ore constituents other than uranium, in the mill tailings, which are retained in tailing ponds on the mill property. The discharge of radioactive materials off the mill site is strictly controlled. Airborne wastes generated during processing include uranium compounds, hydrogen sulfide arsine, or acid gases, all of which require control measures.

2-2 FEED-MATERIALS PLANTS

Following uranium-milling operations the next step in the fuel cycle is additional purification preparatory to the manufacture of fuel elements. Currently this purification is carried out by the Mallinckrodt Chemical Works, Welden Springs, Mo.; National Lead Company of Ohio, Fernald, Ohio; Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tenn.; and General Chemical Division, Allied Chemical Corp., Metropolis, Ill.

Ore concentrates containing approximately 70 wt.% U3Os are processed to remove impurities and to form uranium compounds suitable for feed to the gaseous-diffusion cascades for separation of U235 from U238, for reduction to uranium metal, or for direct use as reactor fuel material. Mallinckrodt and National Lead use solvent extraction and General Chemical uses fluoride volatility to refine the concentrates. After a series of additional chemical steps, uranium tetrafluoride is produced; this is, in turn, converted to uranium metal suitable for reactor fuel-element fabrication.

The liquid wastes from the processing of ore concentrates are radiochemically similar to mill raffinates. They contain less radium and more Th234, however. For each ton of uranium processed, approximately 1000 gal of solvent-extraction raffinates is produced. This